Guest YouTuber Grant Thompson, aka the King of Random, says you can use any type of water – if you’re surviving it’s probably best to get it from a nearby stream than your water bottle – and you need to fill the sandwich bag to about halfway and seal it up.

The other key component is tinder – you need some really good, dry, light tinder, such as flakey tree bark. The gents grind up their tree bark into a fine powder, and use some more tree bark to act as the base.

Next, they twist the sandwich bag of water into as close to a sphere as possible – this is how you make your magnifying glass-like lens.

The reason this works is that transparent spheres act like two lenses working in unison to focus sunlight – the two opposite sides combine to make a biconvex lens like this:

Tamasflex/Wikimedia

As you can see below, you need to find the most perfectly round point in your sandwich bag bubble, and use it to direct sunlight down onto the tinder powder.

Give it a few seconds, and you should start seeing smoke rising out of the pile:

“These double-lenses heat things because light slows down when it passes through them, and then bends with the shape, bending more in the thinner section and less in the thicker section,” Ryan F. Mandelbaum explains for Gizmodo.

“This concentrates the sunlight into a single focal point, all of it adding heat onto the combustible fuel source, in this case, the dried up bark.”

Once you have your smoke, you need to gently start building up your fuel by stacking larger pieces of dry tinder underneath the smokey powder.

A little trick from the King of Random will help you build that smoke to actual fire in your hands.

Obviously if we can start a fire with nothing but a sandwich bag of water and a bit of dried vegetation, we’ve got the chemical process pretty well figured out on Earth.